Francois Hollande, the French president, said yesterday that the Syrian leader had continued to use chemical weapons on the front line, although he added that definite proof had not yet been established.

Officials are examining attacks in the past week on at least three towns where credible reports point to the use of chlorine gas, leaving dozens of people seeking treatment.

While the Syrian regime has handed over 80pc of its declared chemical weapons stockpile for destruction under UN supervision, the attacks appear to show Mr Assad's regime is continuing to use poison against its own civilians.

"We have a few elements of information but I do not have the proof," Mr Hollande said. "What I do know is what we have seen from this regime is the horrific methods it is capable of using and the rejection of any political transition."

PANIC

Gas attacks are seen as a particularly effective tool in provoking panic and fear in rebel-held areas.

Laurent Fabius, the French foreign minister, suggested that intelligence reports were being scrutinised, saying his officials received "indications" of recent chemical attacks that were still being verified.

American and British officials are thought to be examining video footage of Chinese-manufactured chlorine gas canisters rigged with detonators hitting the town of Kafr Zita near Hama.

Activists said the gas was first used during fierce fighting when regime troops appeared to be losing control of the strategic town of Khan Sheikhoun. Since the initial attack on April 11, there had been at least two more chlorine-related incidents in the rebel region.

Two separate attacks were reported last week at Tamanah in Idlib province and Zahraa near Homs. Last month, credible reports of gas attacks emerged from Harasta and Jobar, both near Damascus.

While chlorine is not banned in recognition of its wide range of industrial uses, its deployment as a weapon of war is prohibited under the 1925 Chemical Weapons Convention.

It was first used 99 years ago by German forces during the Second Battle of Ypres in an attack on British and French trenches. At least 30,000 died.

"Chlorine was a pretty good chemical weapon to use in 1915 to achieve battlefield advantages and while it has fallen pretty far down the schedule and lists of most dangerous gases, in 2014 it remains an effective choice in certain places in Syria," said Hamish de Bretton-Gordon, a director of chemical weapons experts SecureBio.

"The evidence from Kafr Zita is pretty compelling and is being examined very carefully by officials."

Damascus has accused Jabhat al-Nusra, the Islamist insurgency group linked to al-Qa'ida, of using the gas.

Eliot Higgins, a British blogger who follows attacks in Syria, said the regime's explanation did not stack up.